The subject invention relates to a crankcase scavenged internal combustion engine of two-stroke type, in which a piston ported air passage is arranged between an air inlet and the upper part of a number of transfer ducts. Fresh air is added at the top of the transfer ducts and is intended to serve as a buffer against the air/fuel mixture below. Mainly this buffer is lost out into the exhaust outlet during the scavenging process. The fuel consumption and the exhaust emissions are thereby reduced.
For engines of the above-mentioned kind usually the air inlet of the cylinder is connected via a restriction valve to the air filter of the engine. The restriction valve is arranged, by means of one or several engine parameters, to regulate the intake of air into the transfer ducts. Below the air inlet the inlet tube of the cylinder is connected to the carburetorurettor, whose air inlet debouches into the air filter to the side of said restriction valve.
In order to keep the height of the engine design down it is preferable that the inlet tube is directed as horizontally as possible. It should however not be angled downwards from the cylinder, since there is a risk that the fuel mixture, at idling, will flow backwards into the carburetor from the cylinder, resulting in uneven engine running or possibly engine stop.
These demands on the inlet tube""s orientation, and combined with that these carburetors of prior art technology often extend a bit more vertically in relation to the diameter of the inlet tube, will result in that the air inlet, located above the inlet tube, after all is located relatively high up in the engine. This leads to problems, especially when the engine is applied for handheld tools, e.g. chain saws or grass trimmers, since they might get a clumsy and unpractical design. Moreover, the high located air inlet would easily restrict the cooling air of the cylinder, with reduced cooling of the cylinder as a result.
In order to minimize these disadvantages the inlet tube is usually directed as horizontally as possible without running the risk of engine stop, and this orientation is not optimal.
The purpose of the subject invention is to solve the above-mentioned problems and to achieve a crankcase scavenged two-stroke engine having such a design that the height of the engine would not lead to any problems when using the engine in handheld engine tools.
This purpose is achieved by an engine of the previous mentioned kind according to the invention, in which the air inlet at least partly extends below the carburetor. Since the connecting ports for fresh air on the inside of the cylinder have a predestinated location above the inlet in order to achieve the desired piston porting, this means that the air inlet has to cross the inlet tube, e.g. by arranging external air ducts, such as hoses or tubes, between the cylinder and the air inlet located below the carburetor.
Often there is at least one connecting port on each side of the cylinder, and via air ducts, which extend passing the inlet tube, the connecting ports can be connected to a common air inlet.
This location of the air inlet is thus completely inverted in relation to known engine design and creates a number of advantages.
In the first place the whole engine design will become more compact thanks to a more optimal use of the space to the side of the cylinder.
In the second place the air is forced to flow through the air inlet from a position below the carburetor and inlet tube, thereby allowing a more favourable flow direction into the engine.
In the third place the cooling air will be given better access to the cylinder since the air inlet is located at a greater distance from the cooling fins. The fact is that the whole air filter can be located at a greater distance from the cylinder, which also improves the inflow of cooling air.
Furthermore, according to a particularly preferred embodiment the air inlet is connected to the cylinder at a mouth below the inlet tube. This is an advantage, since the demand for a connection above the inlet tube will be totally eliminated, thereby the location and orientation of the inlet tube can be chosen more freely. Owing to this design also the demand for external air ducts passing the inlet tube would be eliminated, and a more compact and space-saving solution is created. At the same time the air flow through the air inlet will be improved even more and the demand for sharp bends in the air inlet will be eliminated.
Preferably the inlet tube is directed obliquely upwards in a direction away from the cylinder, and preferably the carburetor is located in the prolongation of the inlet tube. This higher up location of the carburetor in the engine will create more space for the air inlet located below the carburetor. Furthermore, the intake flow of air/fuel-mixture is forced to flow directly downwards into the crankcase, thereby lubricating the piston rod bearing.
The air inlet can extend partly on the outside of the carburetor, so that the restriction valve, which usually is arranged at the end of the air inlet, will be located essentially aligned with the air inlet of the carburetor. This design makes it possible to use a straight filter stud between carburetor and filter, resulting in reduced throttling of the air and improved delivery to the engine.
Owing to the orientation of the inlet tube and the carburetor the restriction valve can be located above the carburetor, or at the same level as it. However, it can as well be located obliquely above or obliquely below the carburetor. In case a common air filter is used this relation will affect the orientation of the air filter, which is made to align with the neighbouring parts in an optimal way.